Hi and welcome to the first guide in the art of Alfa surgery. I'm picking on the common faults which are simple for the home mechanic to fix.
Today a Thermostat on a Twin Spark plastic top. The model in the pictures is a 2000 146 1.6TS.
1. Well I stuck the car on ramps to allow my bucket underneath.
2. Depressurize the system, Simply remove the expansion tank cap slowly incase it sprays.
3. Drain the system. Find a hose connection below the thermostats height level (I chose this one on the engine front rail), diconnect it and drain the fluid. Antifreeze is harmful to the enviroment, so best not to let it go down the drain (that's why the buckets there, plus I reuse mine later as it's only 3 months old).
4. Unplug the temp sensor and disconnest the hoses from the thermostat housing.
5. At this point I chose to remove the temp sensor from the housing as it is needed in the new one.
6. Next loosen and remove the 2 bolts that hold the thermostat housing to the engine head. Clean the head side around the hole trying not to get dirt inside.
7. Time to fit the new one, I loosely fited the tempsensor into the new housing whilst it was on the bench and ensured the seal was seated correctly before siting the thermostat on the head and fiting the 2 bolts.
8. Reconnect ALL the hoses. I replaced all the band clamps that ALFA had fited with worm drive clamps (jubilee clips). These just make life easier.
9. Tighten up the temp sensor and reconnect the plug. Below in the pic you can see the pin pattern and keyway, the pins will bend if you force it in the wrong way round.
10. Refill the coolant. As I said earlier, I reused mine as Iknew it was only 3 months old. Plus catching it in the bucket meant I only had to pour that amount back in after I'd filtered it. But this is a good time to do a water change if your car is due one and a mix of 50:50 ethel Glycol/water will keep you cool. Refit the expansion tank cap and that's it, these engines don't suffer from airlocks in the cooling system and so after running the engine for a while the cooling system will self bleed, just allow the water to cool again and make sure your coolant level is correct.